1. Technical Field
The present invention relates to a control circuit which controls a light-emitting element.
2. Background Art
Currently, in order to dim the light-emission intensity (brightness) when an incandescent lamp is used as illumination, a system is used which controls the light-emission intensity by controlling a conduction angle of an alternating current (AC) power supply and reducing an average value of a current flowing in the incandescent lamp.
On the other hand, in view of energy conservation and the like, the use of a light-emitting diode (LED) as the light-emitting element for illumination in place of the incandescent lamp is desired. When the LED is used for illumination, it is desired to apply the dimmer system for incandescent lamp which is already used in the infrastructure.
FIG. 9 shows a control circuit 100 of an illumination system in the related art. The control circuit 100 comprises a rectifying unit 10, a rectifying capacitor 12, a choke coil 14, a regenerative diode 16, a switching element 18, a reference voltage generating unit 20, and a comparator 22.
When an AC power supply is supplied to the rectifying unit 10, the AC power supply is full-wave rectified. The full-wave rectified voltage is averaged by the rectifying capacitor 12, and is supplied to an anode terminal of the LED 102 as a drive voltage. A cathode of the LED 102 is grounded through a series connection of the choke coil 14, the switching element 18, and a resistor element R1. A terminal voltage of the resistor R1 is input to an inverting input terminal of the comparator 22 as a comparative voltage Vcmp. On the other hand, the reference voltage generating unit 20 comprises a series connection of a resistor R2, a Zener diode ZD, and a resistor R3, and divides the voltage rectified by the rectifying unit 10 and inputs a reference voltage Vref to a non-inverting input terminal of the comparator 22. Based on a comparison result between the reference voltage Vref and the comparative voltage Vcmp by the comparator 22, switching of the switching element 18 is controlled, a current is supplied to the LED 102 through the choke coil 14, the switching element 18, and the resistor element R1, and light is emitted from the LED 102. Here, when the comparative voltage Vcmp is lower than the reference voltage Vref, the switching element 18 is switched ON and the current is supplied to the LED 102, and when the comparative voltage Vcmp becomes larger than the reference voltage Vref, the switching element 18 is switched OFF and the current to the LED 102 is stopped. In this manner, the current flowing to the LED 102 is controlled, and the average light-emission intensity of the LED 102 can be controlled. In addition, the regenerative diode 16 which regenerates the energy stored in the choke coil 14 to the LED 102 when the switching element 18 is switched OFF is provided in parallel to the LED 102 and the choke coil 14.
In the dimmer circuit for the incandescent lamp or the like, the light is dimmed by adjusting a duty cycle of the applied AC voltage, and consequently, current flowing in the incandescent lamp, by controlling a conduction angle of the AC power supply. When the dimmer circuit is applied to a resistive element such as the incandescent lamp, as shown by a dotted line in FIG. 10, the voltage and the current are output in a stable state in approximately the same phase. When the control circuit 100 as described above is applied to such a dimmer circuit, as shown by a solid line in FIG. 10, the input current I in of the control circuit 100 rises with the rise of an input voltage Vin of the control circuit 100, but the input current I in may then be reduced, and an unstable state may be reached where the switching element 18 is repetitively switched ON and OFF.